Processing of raw cellulosic materials



Patented Jan. ii, 193].

UNITED STATES PATENT orl-lca GEORGE RICHTER, F BERLIN, NEW HAMPSHIRE, ASSIGNOR TO BROWN COIPANY,

O F BERLIN, NEW HAMPSHIRE, A CORPORATION OF ELAINE IPROCESSII Q'G 0]? RAW CELLULOSIC IATERIALs 1N0 Drawing.

In the manufacture of cellulose pulp, it is the practice to cook 'r aw cellulosic material such as wood inchemical liquors which react upon and dissolve the substances which bind the fibers together, thereby liberating the fibers. The compositionand characteristics of the resulting pulp depend largel upon the type of chemical liquor employe for fiber I liberation,,-it being possibleto produce pulps of widely varying compositions and charac-' teristics from one type of raw material.

In accordance with the rocess of the pres-- sent invention, I accomp lsh aliberation of cellulose pulp from suitableraw material in a liquor which is initially acid, for instance in a sulphurous acid liquor, andwhich is rendered alkaline during the cooking operation. B such a process, it is possible to produce a ished pul of higher alpha cellulose content than t at present in the usual chemical wood pulps, as cooking in bothacid and alkaline liquors results in a removal from" the raw material of non-alpha cellulose constituents which might otherwise survive cooking in eitherv acid or alkaline 1i uors alone. In carrying out the. process 0 the present invention, the raw material is cooked in a sulphurous acid liquor, preferably short of com lete fiber liberation, as it is thereby possi 1e to avoid the loss in yield and injury to the fibers occurring when the isolated fibers are digested in acid liquors. After cooking to the desired extent has been efiected in the acid 1i nor, the liquor is converted, preferably w e hot, to an alkaline liquor the alkalini of which is afi'orded at least in part, y sodium sulphide. lBy continuing the cooking operation until fiber liberation is complete in an alkaline liquor contag sodium sulphide. it is possible to produce a finished pulp not only of high alpha cellulose content but having physical characteristics superior to those possessed by products pro duced in alkaline liquors in which caustic soda alone is used as a fiber-liberating chemical. The superior characteristics of the tin-- ished pulp are attributable to the dac tion of the sodium sulphide on the fibers, as

compared with alkaiies such ascac soda Ar with the acid gro p d Sul h A salts e S0 lllm- P 1 nor to produce thios'ul hates Application filed September 24; 1929. semi no. $94,931.

It is doubtless-because of the mild action of sodium sulphide'that kraft pulp, so called, which is roduced in sodium-sulphide-containing al aline liquor is superior in its physical characteristics to soda pulp so called, which is produced in a cooking liquor containing practically only caustic soda.

When converting the sulphurousacid liquor to an alkaline condition in accordance with the process of the present invention, the sulphur constituent resent in the liquor is maintained combined in the form ,of watersoluble compounds. This result may bet-realized if the acidliquor is neutralized with suitable alkaline compounds" such as caustic soda or sodium carbonate, as these alkalies react present in the liquorto form water-solub After such neutralization, a suitable amount of sodium sulphide or mixture of caustic soda and sodium sulphide may be added without danger of precipitating sulphur, such as 00- curs when sodium sulphide is added to an acid liquor. The precipitation of sulphur is highly undesirable, in thatit contaminates theresulting ulpvand is diflicult to remove by washing. ile the precipitated sulphur may redissolvein the resulting alkaline the thi pbate, too, is undesirab e, in that under the temperature conditions of cooking it may decompose and precipitate sulphur, which will contaminate the resulting pul unless a sulphidity sufliciently high to dissolve such sulphur is maintained in the liquor, such a sulphidity being so high as to be imprac 'tical and uneconomical.

Acid cooking liquors of various compositions may be employed in the process'o the present invention, dependin upon thechar- 'acteristics desired in-the ished product. When a finished pulp especially low in pentosans is desired, the acid cooking liquor is preferably one containing sulphurous acid sulphuric acid. Evidentl sucha li uor effects more pronounced by olysis of t a penalone or together with another acid such as tosans resent in the raw cellulosic material, I

producing water-soluble reaction products. r

:1. pulp of low pentosan content is 7 2n Blig ;w "(:11 isof comparatively little value in alkav suchas opaqueness and cloudiness to lacquers or films produced from the derivative. If,-

' however, the finished pulpis to be used asa raw material for papermaking,',it may be 'pre'ferable to employ an acid cooking liquor containing S combined in the form of sodi- 4 um sulphite or containing sodium salts such as sodium bisulphate, sodium sulphate, or v a p time than that of'the usual sulphite process, v

- borate, which undergo metathesis in a sulphurous acid liquor to give sodium sul- 5' phite; After cooking in the sulphurous acid I :liquor under the desired conditions for a suitable period of time,itis preferabletorelease Jor'exhaust' free SO,- from the hot liquor, so

' that alkalineednot be consumed in reacting 1 such SO to produce sodium sulphite,

linecooking. v j

1 The spent. acid liquor contains residual sulphurousj acid groups existing as sulphurqus as acid or combined in the form of-loose, complexorganic compounds. :InTorder'to avoid the formation of sodium thiosulphate and the precipitation of sulphur, non-reducing alkaes -su chas' caustic soda and/or sodium car.-

so bonate, may' be' added in amount sufiicient to y neutralize the liquor. Wh e n sodium'carbonate is employed, carbon dioxide is liberated during theneutralizing reaction, so that it is necessary topermit the escape of this gas I through a suitable relief valve- After neutralization has been effected, sodium sulphide and/or caustic soda may be'added to pro- 'duce the desired alkalinity in the liquor, whereupon cooking may be continued in the 40 "resulting alkaline liquor until fiber liberation 1 I has been completed. The alkaline liquor in which fiber liberation is completed may be of sufliciently high alkalinity to produce a finished pulp of high alpha cellulose content,

as say, 92% to 95%, or even higher, although if desiredthe alkalinity of the liquor may be lower, such as to produce a pulp having an gg ayha' cellulose content of, say, only 89% to u; 'A rocess such as described makes possi-' ble the realization of .many advantages, among which may be enumerated the followmg: V a, a 1. It rmits the production of a finished ab-pulp of zhe desired alpha cellulose content with a comparatively small amount of sodium base chemicals, as the initial step of cooking in an acid liquor may be carried out in a liquor containing little, if any, sodium base 00 compounds. Subsequent alkaline cooking may be eflected in an alkaline liquor of a sodium base compound concentration not inexcess of that employed in the usual kraft process, and often ap arently'less.

C6 2.It makes poss1 e the conservation of cess, for example.

" the bottom of thedigeste the heat associated with thcspent, 3

not and cellulosic material after cooking.

ki liq- 3. The step of washing the cellulosic mate- -the finish of the pulping operation.

4. The preliminary neutralization of the spent acid liquor permits cooking in a sodium-sulphide-containing alkaline liquor, and

results in, a fihished-havingfexcellent physical and paperm 'hg characteristics' 5. The entire process need require no more as each of the cooking steps is of short-dun tion, compared with'the usual sulphite pro-' While various-spe ific may be adopted in accordance 'with the present in- J vention, the following example will serve. as

an illustration'offa procedure intended for I l the production of pulp of alpha y'suit:

. cellulose but'lowpentosan content;

able raw cellulosic materiahsuch as s ruce.

chips, may be placed in 'the usu'al' aci sulphite digester, together with asulphurous The digester charge may befconiined and. j l

slowly heated to atemperature of about 250- I F., say in about two hours, at which temperature it v-may be maintained .for a period of may be raised quickly and the relief valve of some cases be carried out until substantially atmospheric pressure is reached and a comparatively small'amount of alkali is neces 'sary to neutralize residual sulphuro'usacid.

groups in the liquor, After the release of sulgimdioxide from the liquor, a suitable al li such as caustic soda may be, introduced into the digester. The amount of caustic soda introduced is based upon the'amount of sulphurous acid groups presentin the liquor in f the digester, this being ascertained by analyzing a sample of the liquor. The caustic soda 7 in the form of a strong solution may be intro duced near the bottom of the digester, so that natural circulation of the liquor will result in a. difiusion of the solution through the liquor, although if indirect cooking is practiced, the liquor may be pumped from r through the usual liquor heater torthe to 'ofthe di r, to ensure a uniform di usion' of t e alkali through the liquor. when sodiumcarbonate is employed as the alkali, carbon dioxide generated through the neutralization reaction may be allowed to escape from the digester through a suitable relief valve. -After the neutralization reaction has been ile rebeing as about two hours, whereupon the temperature completed, the desired amount of alkaliimay be added to the digester, as in the form of a solution of sodium sulphide and caustic soda containing to 18% caustic soda equivalent- The amount of alkali added to the digester may vary, dependingupon the alpha cellulose content desired in the finished pulp.

' If a finished product of, say, about 9.2% alpha cellulose content isdesired, suflic-ient alkaline solution should be added to the digester to produce a liquor containing'1.5 pounds of actiVeNa O per cubic foot. If, however, a product of unusually high alpha cellulose content is desired, the alkali concentration produced in the liquor may correspond to 2.5

pounds of active Na O' per cubic foot. Preferably about 40% or more of the alkali exists in the form of sodium sulphide, as the presenceof sodium sulphide is advantageous,

particularly-in the production of a pulp which is to be finally bleached and employed in papermaking I After the addition of a1 kali, the liquor contains caustic soda, sodium sulphide, sodium sulphate, and possibly slight residues-of sodium. sulphite. The hot liquor together with .the previously cooked chips may then be heated to a'temperature of about 335 F. and maintained at this temrature for two to three hours to complete ber liberation, but this temperature may be varied somewhat, depending upon factors such as the period of cooking and the alkali content of the liquor.

The hot digester contents may be blown as usual from the digester into a pit, and the pul separated from the spent cooking liquor as i y washing. The spent cooking liquor may then be concentrated to the desired consistency, as in multiple eflect evaporators, and

the concentrated liquor then delivered into a furnace, where'th'e organic combustible material is burned and the inorganic sodium constituent is smelted preferably under reducing conditions to produce a smeltconsistlng of sodium sulphide and sodium carbonate.

T The smelted compounds may then be dissolved in water to produce a solution from which sodium carbonate may be selectively crystallized for use in neutralizing other spent acid liquor, or which may be causticized with lime to produce an alkaline liquor containing caustic soda and sodium sulphide.

When sodium carbonate has been selectively" The washed pulp may be bleached, if desired,' in, r to produce a white product suitable for use 'to as a-raw material either for papermaking or for converslon into cellulose derivatlves.

' If the desired cooking step has been carriedout in a liquor containing sodium compounds, it will be seen that the sodium constituent present during both acid and alkaline cooking is accumulated in a single aqueous liquor, from which recovery is efiected. The neutralizing agent being a sodium compound, sulphurous 'acid residues remaining in the spent cooking liquor are changed into sodium sulphur compounds, which, after smelting, appear in the smelt as sodium sulphide, there'- by resulting in the recovery of sulphur constituent, which isordinarily lost. in the water employed for washing cellulosic 'material after acid sulphite digestion. The organic matter extracted from the raw cellulosic material during acid and alkaline cooking is accumulated in a single aqueous liquor, so that when smelting is effected in a combined boiler and smelter furnace, for instance of the general type described in Patent No.

1,326,414, issued December 30, 1919, to Hugh K. Moore and John T.- Quinn, a higher fuel value than ordinarily is realized.

I claim:

H 1. A process which comprisescooking raw cellulosic material in a sulphurous acid liquor, neutralizing the liquor while maintaining the sulphur constituent in combined state, adding alkali including sodium sulphide to the liquor to render it alkaline, and continuing the cooking of the cellulosic material.

2. A process which comprises cooking raw cellulosic' material in a sulphurous acid liquor short of complete fiber liberation, releasing free sulphur dioxide from the hot liquor,

adding non-reducing sodium base alkaline compounds to such liquor to neutralize re.- sldual sulphurous acid groups therein, adding alkali including sodium sulphide to the hquorto render it alkaline, and'continuing the cooking'mof the raw cellulosic material to complete fi r liberation.

3. A process which comprises cooking raw cellulosic material in a sulphurous acid liquor, neutralizing the hot liquor with non- I reducing sodium base alkaline compounds, adding alkali including sodium sulphide to the liquor to render it alkaline, continuing the cooking of the cellulosic material, separating the resulting pulp from the. spent liquor, and recovering the sodium constituent of the spent liquor in the form of alkaline compounds for use in again practising one or more steps of the process.

4. A rocess which comprises cooking raw cellulos1c material in a sulphurous acid'liquor, adding non-reducin sodium base alkaline compounds-to the ot liquor to react upon residual sulphurous acid oups therethe liquor to render it distinctly alkaline,

adding caustic soda and sodium sulphide cbieihqin thecookiiig of the cellulosic m.-

i r' ;sepfiie n r s -pu m e. spent' liquorysmeltingfthe sodium constitu- "entof'the spent liquor in'a reducing atmosphere to produce sodium carbonate and sodium sulphide, causticizing' the sodium carbonate, and using the caustic soda and sodium sulphide for again producing alkalineliquor.-

5 5. A'process' wliich comprises'ooo raw ,cellulosic material in a sulphurous aci liq uor, adding sodium carbonate to the hot li nor to react upon residual sulphurous aci groups therein, adding alkali including sodium sulphide to the liquor to render it alkaline, continuing the cooking of the cellulosic material, separating the resulting ulp from the spent 11 nor, smelting the so 'um constituent of t e spent 1i nor in a. reducing atmosphere to roduce so 'um carbonate and sodium sulphi e, selectively crystallizing sodium carbonate from the smelt for use in again reacting upon residual sul hurous acid groups, and using the rest of t e smelt for again producing alkaline li uor.

In testimony whereof I ave afiixed my signature.

GEORGE A. RICHTER.

" mamas 

